how to make mandatory fields as not mandatory in particular form

how to make mandatory fields as not mandatory in particular form in ax 2009?

Why would you want to do that? Are you talking about system based mandaotry fields or non-mandatory ones you have made mandatory?

System based mandatory fields, In Dimension group if Revision is mandatory i need to make that as not mandatory in all BOM versions

That would be a significant change in dimension group impacting on every transaction, it would mean a mandatory critically required piece of information to impact on planning, BOM etc would be missing. What is the business justification for removal - for me it is a different product, different dimension group where Revision is not valid (remembering Revision is not a standard Dimension group option!!)